You will have less exhaust restriction with a dual system. Gas mileage will usually go up some, I would expect about .7 to 1 mpg better with the two barrel carb. This is a de-tuned smog motor that is probably struggling to get 10-12 mpg so a 1 mpg increase would be significant. Later on, going to a 4 barrel carb, higher compression pistons and a better cam shaft will also help a lot. If you get the horsepower up with the previous modifications, going to a higher gear ratio will also help if your are looking for more economy. You can also play with the timing and mixture settings with the vehicle as is to see if you can get it to run better. The smog settings are not ideal for either power or economy. Be sure your heat riser is working properly as a stuck heat riser will cause the new exhaust to rot out quickly.
Dave
Cam specs on the '72 400 2bbl/4bbl were "wilder" than in the prior 383s, a little more lift and longer duration in order to help make the engine a better air pump. Increased overlap for a little more "internal EGR" during overlap at lower rpm, too.
Observed fuel economy differences between our '66 Newport 383 and the '72 Newport Royal 400 was about 1-1.5mpg less on the 400 16mpg highway vs 17mpg, for example, but it ran well too. Ignition timing specs were similar, too, once past about 1500rpm. After hearing of all of the "doom and gloom" about retarded timing and such, I was surprised when I finally checked the factory specs and found that things were still pretty much optimized, except during the lower rpm levels.
The Holley 2210 is subject to the airhorn deformation from over-torqueing the air cleaner wing nut, so a 4bbl is a good option to get away from that issue. The 2210, though, is a more efficient carb than other 2bbls which Chrysler used back then. When I retrofitted one to our '66 Newport (from the Stromberg WWC3-262, throttle response was better. The first time I drove home from college (about 290 miles) under the 55mph speed limit, I documented right at 20mpg @ that lower cruising speed.
The OEM cam specs for the 400/440 V-8s are more toward the prior 383/335 & 440/375 cam than the older 260/268 .425" lift cam. Adding an Edelbrock dual plane 4bbl, an Edelbrock AVS2 carb (now on sale at Summit Racing!), and a good undercar 2.25-2.50" exhaust system, ensuring the factory electronic ignition system is working "as designed", would make a nice performing vehicle . . . for what it is.
Rear axle ratio? It should already be the 2.76 or 2.71 ratio, which should yield about 28mph/1000rpm with the factory H78-15 or P225.75R-15 tire size. The rear axle ratio should be on a metal tag attached with a bolt on the center section of the axle. Figures for the 3.21 axle would be more like 25.5-26mph/1000 rpm with the same size tire.
If the trans kickdown linkage is still at the factory setting, take a small black plastic wire-tie and insert it into the back of the kickdown rod slot. This should raise part-throttle upshift speeds a few mph. Enough that the trans should get into high gear just prior to 29mph under light acceleration. This small change will make the car stay in the lower gear a bit longer so that when the upshift happens, the torque converter will be more "locked-up" (i.ee., "tight") such that any acceleration happens with "gear" rather than "on the converter". Better than trying to manually shift too. Part-throttle kickdown will happen a bit easier and last a bit longer, too. End result is more acceleration with less throttle input.
The normal adjustment is easy to get to, but the wire tie method works well and easier. PLUS it's reliable and durable. I spent time tweaking the adjustment on our '72 Newport 400 2bbl and my '70 Monaco 383 4bbl, but I discovered it on my '80 Newport 360 2bbl after they moved the adjustment to a place that wasn't easily accessible from the top. On the '66 Newport (threaded adjustment), two more turns of preload did the trick. NO trans durability issues as there was no large change of adjustment, just fine-tuning.
The '72s were nice cars. I like the styling of them more than the later '73s, personally. A GREAT instrument panel design!
CBODY67
